Olivia Rodrigo's first-ever music festival ended with a major surprise ... Stevie Nicks joined her onstage, while a massive donation pushed the event's fundraising total to $20 million.

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Rodrigo closed out Daisy Chain Fields in Southern California Saturday by bringing out the Fleetwood Mac legend for a duet of "Landslide."

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The surprise came after Rodrigo also performed with Sarah McLachlan and Alanis Morissette, who stepped in after Yeah Yeah Yeahs singer Karen O had to bow out for a personal matter.

Nicks later praised Rodrigo from the stage, saying the young Grammy winner reminded her of herself at that age and calling her "very special."

Rodrigo also confirmed the festival isn't a one-and-done event ... telling fans she'll see them again next year.

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Then came the big money news ... Melinda French Gates surprised the crowd by announcing a $10 Million donation to the festival's 10 nonprofit partners.

That effectively doubled the $10 Million Rodrigo had already pledged, bringing the total to $20 Million for organizations supporting women and girls.

Definitely not a bad way to launch a festival.